Recently, as the portability and cordless tendency of instruments have progressed, a demand for a non-aqueous electrolyte secondary battery such as a lithium secondary battery which is small in size and light in weight and has a high energy density, has been increasingly high. As a cathode active material for the non-aqueous electrolyte secondary battery, a composite oxide of lithium and a transition metal or the like (which may be referred to as a “lithium-containing composite oxide” in the present specification) such as LiCoO2, LiNi1/3Co1/3Mn1/3O2, LiNiO2, LiNi0.8Co0.2O2, LiMn2O4 or LiMnO2, has been known.
Among others, a lithium secondary battery using LiCoO2 as a cathode active material and using a lithium alloy or carbon such as graphite or carbon fiber as a negative electrode, can obtain a high voltage at a level of 4 V, whereby it has been wisely used as a battery having a high energy density.
However, in the case of the non-aqueous type secondary battery using LiCoO2 as a cathode active material, further improvement has been desired in the capacity density per unit volume of the positive electrode layer (which may be referred to as a “volume capacity density” in the present specification) and in the safety, and it had the following problems: it was insufficient in terms of the cycle property relating to reduction in the discharge capacity caused by repetitive charge and discharge cycles, in terms of a weight capacity density or in terms of the charge and discharge rate property.
In order to solve these problems, various studies have been conducted heretofore as described below.
For example, in order to improve the discharge capacity and cycle property, it is proposed to use lithium-containing composite oxide particles with at least two peaks in a particle size distribution (cf. Patent Document 1 and Patent Document 2).
Furthermore, in order to improve battery characteristics such as the charge and discharge rate property, cycle property, packing property, or safety, it is proposed to use a composite oxide obtained by mixing a small-particle-size lithium cobalt composite oxide with particle sizes of from 1 to 6 μm and a large-particle-size lithium cobalt composite oxide with particle sizes of from 15 to 22 μm, or to use a composite oxide obtained by mixing a lithium-containing composite oxide with an average particle size of from 5 to 30 μm and a lithium-containing composite oxide with an average particle size of from 0.1 to 10 μm (cf. Patent Document 3 and Patent Document 4).
Moreover, in order to improve battery characteristics such as the cycle property, discharge capacity, charge and discharge efficiency, or safety, it is proposed to use a lithium-containing composite oxide which is obtained by using a solution in which elements are uniformly dissolved, by adding oxalic acid, maleic acid, lactic acid, citric acid or the like in a solution in which raw materials such as a lithium source, nickel source, cobalt source and manganese source are dissolved (cf. Patent Document 5 and Patent Document 6).    Patent Document 1: JP-A-2000-82466    Patent Document 2: JP-A-2002-279984    Patent Document 3: JP-A-2002-93417    Patent Document 4: JP-A-2004-182564    Patent Document 5: JP-A-2000-128546    Patent Document 6: JP-A-2006-93067